Gas detecting element and method of making it

ABSTRACT

A gas detecting element which is formed of a semiconductor material having electrodes embedded therein and wherein at least one substance is added to the material which produces silica or silica gel when oxidized, heated or hydrolyzed.

Unite States Patent Taguchi [54] GAS DETECTING ELEMENT AND METHOD OF MAKING IT lnventor: Naoyoshi Taguchi, 6-8, 2-chome, Hyuga,

Torum-ku, Kobe, Japan Filed: Nov. 17, 1970 Appl. No.2 90,271

Foreign Application Priority Data Dec. 19, 1969. Japan ..44/l028 27 us. (:1 ..317/230, 338/34 Int. Cl. .l ..H0lg 9/00 Field 61 Search ..338/34; 317/262, 230, 231,

[ 1 Feb. 22, 1972 [56] References Cited UNITED STATES PATENTS 2,285,633 6/1942 Venable ..338/34 3,045,198 7/1962 Dolan-et al. ..338/13 3,186,225 6/1965 Freeman, Jr. et al. .....338/l3 x 2,715,667 8/1955 Auwarter ..317/231 x Primary Examiner-James D. Kallam Attorney-Eugene E. Geoffrey, Jr.

[57] ABSTRACT A gas detecting element which is formed of a semiconductor material having electrodes embedded therein and wherein at least one substance is added to the material which produces silica or silica gel when oxidized, heated or hydrolyzed.

5 Claims, 3 Drawing Figures GAS DETECTING ELEMENT AND METHOD OF MAKING IT This invention relates to a gas detecting element, especially a gas and smoke detecting element including semiconductor which changes its electroconductivity when it adsorbs gas or smoke, and a method of making the like.

Gas and smoke detecting elements comprising metal oxide semiconductor materials which change their electroconductivity when they adsorb gas or smoke have been well known in the art and can be divided into two groups, one being that of the reduction type and comprising SnO ZnO, Fe O- or the like and the other being of the oxidation type and comprising NiO, Cr o or the like. The most of such elements have been made by sintering those materials as the above at very high temperatures in order to provide them with high mechanical strength. However, when the elements were sintered at high.

temperatures, they have lost much of adsorption areas and often undesirably reduced their gas detection sensitivity remarkable. While theabove-mentioned semiconductor materials can be formed into and used as a thin film, such thin film type detecting elements are inferior to the above-mentioned sintered type in their gas detection sensitivity and not usable practically.

Therefore. an object of this invention is to provide a gas detecting element of metal oxide semiconductor, which is mechanically strong and rigid and has a high gas detection sensitivity.

Another object of this invention is to provide an improved method ofmanufacturing such gas detecting elements.

According to this invention, provided is a novel gas detecting element comprising a metal oxide semiconductor material and silica or silica gel formed therein. In manufacture of the gas detecting element of this invention, the metal oxide semiconductor material is added with a substance or substances which can be transformed into silica or silica gel by the succeeding treatment.

It is well known that silicon and silicon compounds are easily transformed into silicon oxide SiO when heated in air. Especially, organic silicon compounds, such as alkyl silicide, silanol and silane diol, which are derived from silicon chloride SiCl silane Sil-L, and the like, easily hydrolyze to produce silica gel or silica when heated in air. This product has high gas permeability and serves a function of improving mechanical strength of the element and minimizing its time rate of change of gas detection sensitivity. According to an embodiment of this invention, a sintered metal oxide semiconductor element is impregnated with silicate hydrosol and then heated to transform the silicate hydrosol into silica gel. The resultant element is rigid and hard and can withstand vibration, shock, scratching and other severe external conditions.

Other features of this invention will be more clearly understood from the following description with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a schematic sectional view representing an embodiment of gas detecting element according to this invention;

FIG. 2 is a schematic sectional view representing a second embodiment of gas detecting element according to this invention; and

FIG. 3 is a schematic circuit diagram representing an embodiment of alarm device including the'gas detecting element of FIG. 1.

Throughout the drawings, like portions are indicated by same reference numerals.

Referring now to FIG. 1, the gas detecting element of this embodiment includes a semiconductor body 1, a pair of electrodes 2 and 3 embedded therein and a spacer 4 placed between the both electrodes. The body 1 is composed ofa sensitive metal oxide semiconductor material 5 and crack preventing particles 6 such as powdered alumina or quartz. The electrodes 2 and 3 are composed of straight or coiled wires of gold, platinum, iridium, palladium, nickel or chromium or an alloy thereof. While the electrodes 2 and 3 may be different in material and geometry from each other, it is convenient that the both are perfectly identical in material and geometry for selection of the electrodes. In the illustrative example, the electrodes 2 and 3 are identical and respectively composed of a 0.09-millimeter wire of palladium-iridium alloy consisting of percent Pd and 20 percent Ir having a coiled portion of 0.6 millimeter inner diameter and 12 turns. The value of resistance of the coiled electrode is 2 ohms. The spacer 4 may consist of ceramic or glass and in this example it is shaped as a rectangular parallelepiped of 1.5X2X3 mm.

In the manufacture of the detecting element illustrated above, the electrodes 2 and 3 are placed on the both faces of the spacer 4 so as to contact therewith and then heated by a current flowing therethrough at about 1,000 C. to fuse both faces of the spacer to the contact portions of the coiled electrodes.

Powdered tin oxide SnO having particle size of about 1 micron and containing, 0.3 percent by weight of palladium is mixed with a same weight of IOO-meshaIumina or quartz powder and the mixture is kneaded with water. The resultant composition is coated on the above-prepared electrode-spacer structure as shown in FIG. 1 to form the sensitive body I. Then the body is dried in air and thereafter heated by a current flowing through the electrodes 2 and 3 at about 700 C.

Seventy-five milliliters of tetraethyl silicate diluted with 25 milliliters of water is added with 0.3 milliliters of hydrochloric acid and stirred for 30 minutes to obtain a transparent solution. This is silicate hydrosol which is transformed into silica gel when dried as it is.

Then the sensitive semiconductor body I prepared as in the above is dipped in this solution for about 10 seconds and thereafter slowly heated up to 600 C. in the same manner. After cooled in the air, the body I exhibits very high mechanical strength and improved detection sensitivity.

FIG. 2 shows a small-sized gas detecting element which has a special utility in a portable alarm device or detector which is required to be used with a small current from a battery. The spacer 4 in FIG. 1 is omitted from this element. Since the body 1 shrinks freely when sintered because of the omission of the spacer 4, the crack preventing particles 6 in FIG. 1 can be omitted as shown in FIG. 2. The electrodes in this embodiment are preferably made of twisted fine gold wire. According to this embodiment of this invention, an element having a diameter less than 1 milliliter can be easily manufactured.

The gas detecting element of FIG. I manufactured in the manner as described in the above was tested in comparison with a similar element which had not been dipped in silicate hydrosol by use of an alarm circuit as shown in FIG. 3. In the drawing, the primary winding of a transformer 7 is connected to a commercial l00-volt AC supply. The both ends of one electrode 2 of the gas detecting element 1 are respectively connected to one end and an intermediate tap of the secondary winding of the transformer 7 and the other end of the secondary winding is connected through a buzzer 8 to one end of the other electrodes 3 whose other end is kept free. The buzzer 8 in this test was selected to have impedance of 4 kilohms and its enabling voltage was set at 40 volts. The transformer 7 was so selected that one volt is applied to theboth ends of the electrode 2 and volts is derived from the both ends of the secondary winding. The voltage across the buzzer 8 was measured as 10 volts in a clean air and 70 volts in an air containing isobutane at 0.1 percent concentration. On the contrary, in the case of an element which is the same as the present element but was not dipped in silicate hydrosol, this voltage was measured as 8 volts in a clean air and 24 volts in the same isobutane-containing air. Since this voltage across the buzzer is proportional to the gas detection sensitivity of the gas detecting element, it is apparent that the element according to this invention is extremely sensitive in comparison with the element according to the prior art. Moreover, this voltage of the present element was measured as 70:2 volts in the above isobutane-containing air after continuous use for 100 days. Therefore, it is also apparent that the time rate of change of the sensitivity is very small.

As described in the above, according to this invention, it is possible to increase mechanical strength of the element remarkably to raise detection sensitivity thereof and to minimize the time rate of change of sensitivity. This remarkably raises reliability and the worth of utility of gas detecting elements of this kind.

The above examples are only for an illustrative purpose and various modifications and changes can be made without leaving the spirit and scope of this invention as claimed in the appended claims. For example, silicate hydrosol may be hydrolyzed by concentrated sulfuric acid in place of heating, if there is no trouble in spending a relatively long time. While the presintered element was impregnated with silicate hydrosol in the above example the semiconductor material can be primarily kneaded with silicate hydrosol. in this case, however; it is desired that only a desired amount is prepared at each time since gelation occurs automatically.

What is claimed is: i

1. In a method of manufacturing a gas detecting element in- .cluding assemblying a mass of metal oxide semiconductor and to said body, a material convertible to silica upon heating, ox-

idation or hydrolization thereof.

2. The method according to claim 1 wherein the material applied to said body includes at least one organic silicon compound.

3. The method according to claim 1 wherein the material applied to said body includes at least one compound selected from the group consisting of alkyl silicides, alkyl silicates and silanols.

4. The method according to claim 1 including the step of heating said assembled mass.

5. A gas detecting element comprising a self-supporting body formed of metal oxide semiconductor and insulative particles with electrical terminal means, said body further including a material which has been converted to silica upon heating oxidation of hydrolization thereof. 

2. The method according to claim 1 wherein the material applied to said body includes at least one organic silicon compound.
 3. The method according to claim 1 wherein the material applied to said body includes at least one compound selected from the group consisting of alkyl silicides, alkyl silicates and silanols.
 4. The method according to claim 1 including the step of heating said assembled mass.
 5. A gas detecting element comprising a self-supporting body formed of metal oxide semiconductor and insulative particles with electrical terminal means, said body further including a material which has been converted to silica upon heating oxidation of hydrolization thereof. 